1. Field of the Invention
The present invention relates to an image pickup apparatus including a photoelectric converter.
2. Related Background Art
Conventionally, a solid-state image pickup device has been proposed that instead of reading out a signal charge itself generated for each pixel, converts the signal charge for each pixel into a voltage or a current, amplifies the voltage or the current, and reads out the obtained signal voltage or current through a scanning circuit. This image pickup device is called an amplification-type solid-state active pixel sensor. FIG. 1 is a diagram showing an amplification-type MOS sensor, which is a conventional solid-state active pixel sensor.
In FIG. 1, signal charges accumulated by photodiodes 1 in cells are read out as voltages by amplifying transistors 2 to vertical signal lines 8. Since the amplifying transistors 2 and load transistors 9, which are constant current sources, form a source follower circuit, voltages corresponding to the amount of the signal charges on the photodiodes 1 are read out from the vertical signal lines 8. Each of the cells includes a photodiode 1, a reset transistor 4 for resetting the photodiode 1, a selection MOS transistor 3 for selecting the photodiode 1 whose signal charge is read out to the vertical signal line 8, and an amplifying transistor 2.
In the solid-state image pickup device wherein the above-described cells are two dimensionally arranged, fixed pattern noise is generated that corresponds to fluctuation of the threshold voltage of the amplifying transistor 2, and the image quality is deteriorated. Thus, various noise canceling circuits have been proposed. The structure and the operation of a noise canceling circuit will now be described while referring to the timing chart in FIG. 2. When a pulse 101 is applied to a selected signal line 6-1, extending from the vertical shift register 5, and the MOS transistor 3 is rendered active, the rows of amplifying transistors 2-1-1, 2-1-2, . . . are activated, and output signal voltages which correspond to signal charges accumulated by the photodiodes 1-1-1, 1-1-2, . . . are read out to the vertical signal lines 8 (8-1, 8-2). During a period wherein the pulse for activating each cell of the solid-state image pickup device is at level “H” (pulse 101), the voltage “H” (pulse 102) is applied to the gates of clamp transistors 11 (11-1, 11-2, . . . ), the clamp transistors 11 are turned on, and vertical signal lines 15 (15-1, 15-2, . . . ) are clamped at a clamp voltage 24.
Thereafter, the voltages of the photodiodes 1 (1-1-1, 1-1-2, . . . ) are reset by applying the voltage “H” (pulse 104) to reset signal lines 7 (7-1, 7-2, . . . ). This reset voltage appears on the vertical signal lines 8 (8-1, 8-2, . . . ), and is transmitted by clamp capacitors 10 (10-1, 10-2, . . . ) to the vertical signal lines 15 (15-1, 15-2, . . . ). Since the base voltage for each pixel equals the clamp voltage, fluctuation in the threshold voltages of the MOS transistors can be suppressed. Thereafter, when sample-hold transistors 12 (12-1, 12-2, . . . ) are turned on, a signal is transmitted to vertical signal lines 16 (16-1, 16-2, . . . ), following which a horizontal selection transistor 14 (14-1, 14-2, . . . ) is selected by a selection pulse 105, 106, . . . from a horizontal shift register 19, and a signal voltage for a selected row is read out.
As is described above, since only the voltage change on the vertical signal line 8, caused after the photodiode 1 is reset can be read out to the vertical signal line 16, the affect of fluctuation in the threshold voltage of the amplifying transistor 2 can be suppressed. Especially since the noise element is removed from the output voltage of each solid-state image pickup device that nullifies the affect of the fluctuation in the threshold voltages, only a signal element from which a fluctuation has been removed can be obtained on the horizontal output line.
Further, disclosed in Japanese Patent Application Laid-Open No. 8-18866 is a configuration wherein a current mirror is employed for the load transistor that is the constant current source in FIG. 1. Specifically, the solid-state image pickup device comprises multiple read transistors for reading out a charge obtained by photoelectric conversion to a signal line, and current control means for controlling a current flowing across the constant current source at a time other than when the reading operation is performed by the read transistors. A current mirror circuit is formed for the load transistor, which is the constant current source, to reduce the power consumption when the load transistor is used. However, in this application, an increase in the speed of the processing for the reading circuit is not specifically described.
In the prior art, the amplifying transistors 2 of the sensor cells must drive the clamp capacitors at high speed to rapidly read out the signal output by each sensor cell. Further, in order to increase operation speed, the output impedance of a source follower circuit which is constituted by the amplifying transistor 2 and the load transistor 9, must be reduced appropriately.
Therefore, the ratio (W/L) of the gate width (W) and the gate length (L) of the amplifying transistor 2 must be increased, and a bias drain current for the amplifying transistor 2 must be increased. Since an amplifying transistor 2 is provided in each pixel, an increase in the gate width is not preferable because the chip size of the solid-state image pickup device is increased. Further, an increase in the bias drain current by the load transistor 9 is also a problem because it is accompanied with an increase in power consumption.
In addition, when the clamp capacity is reduced, the above problem does not occur, and high-speed driving is enabled. But when the capacitance of the clamp capacitor is reduced, random noise that is generated by the sensor cell and the reading circuit including the clamp capacitor, is proportional to √{square root over (1/C)} (C is the capacitance of the clamp capacitor) (no explanation for this is given here). Thus, random noise is increased. Further, when the clamp capacitance is reduced, the signal is easily affected by parasite capacitance that depends on the layout of the chip, and the fluctuation in the sensor signals is increased.